Hydraulic fracturing is a technical operation designed to enhance oil and gas production. The process normally involves two steps. First, a hydraulic fracture is formed by injecting a fracturing fluid into the well bore at a sufficient rate and pressure to cause the formation cracked thereby allowing the fluid enter and extend the crack further. Next, a solid propping agent or proppant is carried and placed into the formation to “prop” the fracture open and provide a conductive channel for oil and gas. Oil and gas reservoirs with a low capacity for the flow of fluids usually require hydraulic fracturing to make them commercially viable.
Various types of materials can be used as propping agents, such as sand, resin-coated sand, and man-made ceramic proppant, depending on the type of permeability or particle strength needed. A good propping agent must have sufficient strength to resist crushing by the closure stresses of the formation. The deeper the well depth, the stronger the proppant needs to be to resist crushing. The most commonly used proppant is sand, due to its vast availability and low cost. However, for deeper application, sand does not have sufficient strength to resist crushing due to the closure stresses of the formation, and the permeability of sand is often inadequate.
Sintered bauxite, a high-density proppant with apparent specific gravities about 3.50 g/cc, having an alumina content of about 83% is recommended for use in well depths of greater than 15,000 feet.
Intermediate density proppant, e.g., apparent specific gravity from about 3.10 g/cc to 3.45 g/cc, has been found to have sufficient strength to provide adequate permeability at intermediate depths and pressures, and is strong enough to withstand crushing at well depths from about 8,000 to about 12,000 feet.
However, both high-density and intermediate-density proppants require large pumping equipment, high viscosity fracturing fluids and high pumping rates to keep them in suspension during the fracturing operation, and cause greater than normal wear on fluid carrying and pumping equipment.
Because of the disadvantages of high-density and intermediate-density proppants, the efforts to get a lighter density and crush-resistant proppant by using lower alumina material have been taken since 1980s. Additionally, more and more slickwater fracturing and horizontal well applications require lightweight ceramic proppants.
A low density proppant is described in U.S. Pat. No. 4,522,731 and U.S. Pat. No. 5,120,455 to Lunghofer, using kaolin clay having a 50% alumina content. This low density proppant has an apparent specific gravity of less than 3.0 g/cc.
Another lower density proppant, having an apparent specific gravity of from 2.20 g/cc to 2.60 g/cc, is described in U.S. Pat. No. 5,188,175 to Sweet, using raw material with an alumina content of from 25% to 40%.
An even lower density proppant is described in U.S. Pat. No. 7,036,591 to Cannan, having the alumina content between about 40 and 60%, for use in shallow oil and gas wells. To achieve the specific gravity of from about 1.60 g/cc to about 2.10 g/cc, the firing time of the pellet is less than about 30 minutes and the goal is to sinter and obtain crystallization without fully densifying the proppant.
Since most of fracturing operations are in the low depth range where sand is used, it is desirable to provide a lightweight proppant which can be produced from inexpensive, low alumina content, e.g. a naturally occurring clay with alumina content less than 25% and which exhibits conductivity greater than sand when used in low or medium depth applications.
Reducing the alumina content of the particle generally reduces its density. However, there is a real problem with the strength of the proppant when the alumina content gets too low. For this reason, the goal of the industry has been set to develop a low density proppant without sacrificing strength.
The ultra-lightweight proppant of the present invention uses naturally occurring clays, such as porcelain clay, kaolin (or flint clay) as main raw materials, and are less expensive than bauxite. Therefore, it has lower manufacturing cost per pound. Moreover, the naturally occurring clays have more availability than bauxite.
In addition, from end-user's point view in hydraulic fracturing, fewer pounds of ultra-lightweight proppant are required to fill a crack in the formation for a given propping application because of its lower bulk density as compared to high density or intermediate density proppants. This is a significant advantage since proppants are generally sold by weight.
Other advantages of the ultra-lightweight proppant include its capability of using a lower viscosity fracturing fluid, being employed at more economical pumping rates, and causing less wear on fluid-carrying and pumping equipment than those required by high or intermediate density proppant.